Colossal and fully reversible barocaloric effect for sustainable cooling in a pressure-sensitive spin-crossover complex

Barocaloric refrigeration has great potential to replace traditional vapor-compression systems, which are conspicuously harmful to the environment. Investigating barocaloric effect driven by a practical pressure remains challenging. Here, we report that the spin-crossover (SCO) complex Fe(PC4HH)2 (PC4HH = 4-hydroxy-N′-((pyridin-2-yl)-methylene)-benzohydrazide) exhibits a colossal and reversible barocaloric entropy change (ΔSr, up to ~125 J·K−1·kg−1) under a small pressure ∼7.5 MPa owing to the extremely high pressure sensitivity of transition temperature (dTc/dP, 5.56 K/MPa). Synchrotron radiation PXRD demonstrate that the source of the enhanced ΔSr and the ultra-large dTc/dP come from the gas (air) adsorption of pressure transimitting medium. The superior pressure sensitivity and outstanding barocaloric performance provide a feasible way to exploit novel barocaloric materials by utilizing the complexation properties of SCO materials with gas. Barocaloric refrigeration has great potential to replace traditional vapor-compression systems, which are conspicuously harmful to the environment. Here, the authors report that pressure-induced spin-crossover transitions in an iron complex can produce a colossal and reversible barocaloric entropy change under a small pressure.